Self-cleaning ceramic layers for baking ovens and method for production of self-cleaning ceramic layers

ABSTRACT

The invention concerns a method for producing a highly-porous ceramic layer and for application of this layer onto metallic, ceramic, enameled and/or glass substrates using porous, ceramic particles, preferably aluminum oxide, titanium oxide and zircon oxide, and an inorganic binder system. The inorganic binder system contains at least one ceramic nanoparticle of a particle size of less than  100  nm, preferably less than 50 nm and particularly preferred less than 25 nm, the solvent being water. Layers produced in this fashion are suited as self-cleaning catalytically active layers e.g. in ovens, in combustion engines etc. or for general coating of substances to considerably increase their specific surface e.g. for catalytic substrates.

BACKGROUND OF THE INVENTION

[0001] The invention concerns a method for producing a highly-porous,ceramic layer which can be applied to metallic, ceramic, enameled and/orglass substrates using porous, ceramic particles, preferably aluminumoxide, titanium oxide and zirconium oxide and an inorganic bindersystem. The inorganic binder system contains at least one ceramicnanoparticle of a particle size of less than 100 nm, preferably lessthan 50 nm and particularly preferred less than 25 nm, the solvent beingwater. Layers produced in this fashion are suited for self-cleaningcatalytically active layers e.g. in ovens, in combustion engines etc. orfor general coating of substrates, to considerably increase theirspecific surface area e.g. for catalytic applications.

[0002] Ovens contain a cooking chamber which is lockable by a door andis delimited by an oven muffle. During roasting and baking, the sidewalls of the cooking chamber are soiled e.g. by splashing fat or meatjuice or the like. This soiling during baking and roasting cannot beprevented. For this reason, the manufacturers have proposed several waysto clean the walls, top and bottom, i.e. the inner space of the cookingchamber. One generally differentiates between catalytic and pyrolyticcleaning.

[0003] For pyrolytic cleaning, the cooking chamber has so-called grillrods which can be controlled and heated through a separate,electronically controlled program and are preferably mounted to the topof the cooking chamber. The organic soiling is carbonized, i.e.completely burnt, at temperatures of more than 500° C. (Cepem Cie EuroEquip Menager [FR2605391] or Bosch Siemens Haushaltsgeräte GmbH[DE2526096]. Pyrolytic cleaning is demanding and expensive due to therequired high temperatures. Ovens with pyrolytic cleaning must havesuitable protective mechanism to block the door of the cooking chamberduring pyrolysis (from approximately 320° C., Bosch Siemens HausgeräteGmbH [EP 0940631] to protect the oven from improper operation. Sincethese ovens furthermore require more expensive heating elements to beable to control the high temperature at all, pyrolysis systems have beenestablished only in ovens of the top price bracket.

[0004] In view of the costs, the catalyst systems are preferable topyrolysis systems since catalytic combustion of soiling takes place atlower temperatures, i.e. below 500° C. Matsushita Elec. Ind. Co. Ltd.[JP03056144] proposes lining of the interior of the oven with acatalytically active coating which consists of a binder system and acatalytically active powder. Metal oxides are used as catalyst,preferably manganese dioxide and silicon resins are used as binders.This catalytic coating permits cleaning of the oven interior alreadybetween 380° C. and 400° C. as stated by the manufacturer. Mixture of acatalyst and a binder 15 system or a layer matrix for coating the innersurface of an oven can also be found with other oven manufacturers.Toshiba [JP60147478] uses manganese oxide or ferrite as catalyst andsodium silicate as binder phase. In an analog fashion, Sharp KK[JP54135076] uses quartz sand or sodium silicate as binder phase andiron oxide or copper oxide as catalyst. These protective rights give nostatement about the effectivity of the two latter catalytic coatings.The onset temperature of the coating, i.e. the temperature at which thelayer starts to work, was reduced in accordance with the above-mentioneddocuments to 270° C. to 300° C. (Toshiba) and even to 250° C. (SharpKK). In practice this means, that there are catalytic coatings whichstart to break down fat etc. in the interior of the oven at temperaturesbelow 320° C., however, the efficiency of the coating is not sufficientto completely finish this decomposition. After each baking or roastingcycle, residues of non-decomposed fat remain in or on the layer liningthe interior of the oven, such that after a very short time, thefunction of the layer is impaired since it is varnished. For completedecomposition, these systems still require temperatures of usually morethan 380° C.

[0005] Finally, NGK Insulators Ltd [JP56095022] should be mentioned,which use manganese oxide, copper oxide and iron oxide as catalysts, anda porous enamel as layer matrix to increase the amount of appliedcatalyst, as well as the protective rights of Matsushita [JP02069574],Cie Euripeenne pour L'Equ [FR2040822] and Hoover LtD [GB1177434] whichall use flouropolymers as carrier layer for the catalysts to minimizethe surface energy of the carrier layer and prevent adhesion.

[0006] Pyrolytic cleaning is very effective at temperatures above 500°C. but is expensive due to the facts given by process technology. Thesesystems are currently used only for ovens of the top price bracket(maximally 10% of all ovens). The reduction in cost promoted thedevelopment of catalytic cleaning. The inner walls of the cookingchamber are thereby lined with a layer which always contains a catalyst.Suitable catalysts are manganese oxide, iron oxide and copper oxide,wherein temperature-resistant polymers, sodium silicate, quartz sand andenamel are used as binder phase of the catalyst or as layer component.The catalysts operate at temperatures of more than 380° C. whichrequires safety measures producing additional costs. Only a fewcatalytically active coatings are known whose onset temperature, i.e.start of fat disintegration in the layer is between 250° C. and 350° C.In these cases, large amounts of residues remain in or on the layerduring permanent operation of the oven below 350° C. with theconsequence that these oven inner coatings varnish very quickly.

[0007] Catalysis is subject to thermo-dynamic rules. A catalyst cannotchange the thermodynamics of a system but only lower the activationenergy, i.e. the tendency to start the reaction. Although combustion ofthe organic soiling occurs thermodynamically only at a highertemperature, it starts at a lower temperature if initiated by acatalyst. Not all parts of the organic soiling disintegrate at this lowtemperature, which leaves residues which cause varnishing of theinterior of the oven with the consequence that the optic and hapticappearance of the oven interior drastically deteriorates after only afew baking and roasting cycles.

[0008] It is the underlying purpose of the invention to develop acoating for the interior of an oven which automatically eliminates thesoiling produced through roasting and baking, i.e. through applicationof a temperature of considerably less than 320° C., wherein the workingtemperature of the layer is preferably 250° C.

SUMMARY OF THE INVENTION

[0009] This object is achieved by a ceramic composition (mass), amixture of a porous ceramic powder and an inorganic binder system, whichcomprises at least one porous ceramic powder with a primary particlesize of between 1 nm and 500 m, preferably between 50 μm and 150 μm andan inorganic binder system which contains at least one nano-scaleparticle.

[0010] In this fashion, porous ceramic layers can be produced havinghigh temperature stability and abrasion resistance. These layers containlarge pores/pore volumes which are accessible for organic soiling (e.g.fats), and also small pores through the introduced porous ceramicparticles, which are not accessible for organic soiling. The porousceramic layers have a very high suction capacity and transport theorganic soiling (e.g. fat and meat juice) initially inside the inventivelayer. The soiling is spread, i.e. distributed on a very large surface.At a temperature of 250° C., the soiling is almost completelydisintegrated with no catalyst being contained in the layer. Precisematching of the binder system and the fact that at least onenanoparticle is used as binding phase, produces a very large innersurface, preferably larger than 20 m²/g, particularly preferred largerthan 70 m²/g and particularly preferred larger than 120 m²/g which isloaded with organic soiling. On the other hand, the reaction partneroxygen which is required for combustion, is stored already in the porousceramic parts, similar to a reservoir and is directly available suchthat the oxidative combustion of the soiling is initiated early andcarried out in quantity already at 250° C.

[0011] First-time production of a self-cleaning layer for ovens isachieved, which removes almost in quantity organic soiling attemperatures of considerably less than 380° C., preferably considerablyless than 320° C. A new possibility to clean ovens consists in theproduction of an active ceramic layer without catalyst, which, however,offers the possibility to spread organic soiling on a very large surface(due to the nanoparticles) and provide the reaction partner, requiredfor oxidation, in the form of a reservoir in the layer. Compared tocommercially available catalytic cleaning systems, the inventiveself-cleaning layer is moreover characterized by a considerably higherefficiency at lower temperatures, preferably between 280° C. and 250° C.preventing early varnishing of the coating.

[0012] The inventive ceramic layer is characterized by the presence ofnumerous pores of different sizes and a high inner pore volume. Togenerate these pores, the inventive composition (mass) preferablycontains two different ceramic powder s particles and particularlypreferred three different ceramic powder particles. The ceramicparticles used are, in particular, chalcogenide, carbide or nitridepowders, wherein at least one of these powders is nano-scale. Thechalcogenide powders may be oxide powder, sulfide powder, selenidepowder or telluride powder, wherein oxide powder is preferred. Anypowder which is conventionally used for powder sintering, can be used.Examples are (optionally hydrated) oxides such as ZnO, CeO₂, SnO₂,Al₂O₃, SiO₂, TiO₂, In₂O₃, ZrO₂, yttrium-stabilized ZrO₂, Fe₂O₃, Fe₃O₄,Cu₂O or WO₃ as well as phosphates, silicates, zirconates, aluminates andstannates, carbides such as WC, CdC₂ or SiC, nitrides such as BN, AIN,Si₃N₄, and Ti₃N₄ corresponding mixed oxides such as metal-tin-oxides,e.g. indium-tin-oxide (ITO). Moreover, also mixtures of the statedpowder parts can be used.

[0013] The inventive composition (mass) contains a ceramic powder whichis characterized by a high specific, largely inner surface, larger than50 m²/g, preferably larger than 100 m²/g, and particularly preferredlarger than 150 m²/g. This porous ceramic powder has an average particlesize distribution of more than 500 nm, preferably larger than 1 μm andparticularly preferred larger than 30 μm. This ceramic powder is anoxide, hydroxide, chalcogenide, nitride or carbide of Si, Al, B, Zn, Zr,Cd, Ti, Ce, Sn, In, La, Fe, Cu, Ta, Nb, V, Mo or W, particularlypreferred of Si, Zr, Al, Fe and Ti. The use of oxides is particularlypreferred. Preferred inorganic solid particles are aluminum oxide,boehmite, zircon oxide, iron oxide, silicon dioxide, titanium dioxide,silicates, stone powder, perlites and zeolites or mixtures of theseinorganic solids.

[0014] The inventive composition moreover contains an inorganic bindersystem, which consists of a solvent and at least one nano-scale powder.The primary parts of the nano-scale powder may be present in anagglomerated form, preferably in a non-agglomerated or substantiallynon-agglomerated form. Any conventional alcohols can be used as solvent,preferably 2-butoxy ethanol, ethanol, 1-propanol, 2-propanol,particularly preferred water. The ceramic powder is an oxide, hydroxide,chalcogenide, nitride or carbide of Si, Al, B, Zn, Zr, Cd, Ti, Ce, Sn,In, La, Fe, Cu, Ta, Nb, V, Mo or W, particularly preferred of Si, Zr,Al, Fe, and Ti. The use of oxides is particularly preferred. Preferredinorganic nano-scale solid particles are aluminum oxide, boehmite,zircon oxide, iron oxide, silicon dioxide, titanium dioxide, andgoethite or mixtures of these inorganic nano-scale solids. To adjust theviscosity of the inorganic binder system, all conventional inorganic andorganic acids and lyes can be used, preferably hydrochloric acid,phosphoric acid, sulfuric acid and nitric acid.

[0015] A third ceramic powder may be added to the inventive composition,optionally for precise adjustment of the porosity. This powder consistsof ceramic particles of an average particle size distribution of between10 nm and 1 μm, preferably between 150 nm and 600 nm. The substance ofthe third ceramic powder is oxide, hydroxide, chalcogenide, nitride orcarbide of Si, Al, B, Zn, Zr, Cd, Ti, Ce, Sn, In, La, Fe, Cu, Ta, Nb, V,Mo or W, particularly preferred of Si, Zr, Al, Fe and Ti. The use ofoxides is particularly preferred. Preferred inorganic solid particlesare aluminum oxide, boehmite, zircon oxide, iron oxide, silicon dioxide,titanium dioxide, silicates, and stone powder.

[0016] The inventive composition can optionally be extended throughadding one or more coloring inorganic components. Any conventionalinorganic colorants can be used as coloring component, preferablyspinels. The combination of several coloring components permitsarbitrary adjustment of color effects (patterns and spots) in additionto pure colors.

[0017] The third optionally used ceramic powder is mixed with thelikewise optionally used coloring powders and slurried with the solvent.The porous ceramic powder and the inorganic binder system are added tothis slurry, thereby producing a ceramic suspension which can beapplied, dried and subsequently compacted into a porous ceramic layerthrough spin coating, dip coating, immersion, flooding or preferablyspraying onto a desired substrate. For compacting, temperatures of up to1200° C. can be used, preferably between 400° C. and 1000° C. andparticularly preferred between 700° C. and 850° C.

[0018] The inventive ceramic composition permits application of porous,ceramic layers onto metal, glass, enamel or ceramic surfaces havinglayer thicknesses of between 20 μm and 1 mm, preferably between 70 μmand 600 μm.

[0019] In a particular embodiment of the invention, these porous,ceramic layers can be covered with catalysts to permit utilization ofthese layers for catalytic reactions, e.g. in the chemical industry.

[0020] The following example explains the invention without limiting it:

EXAMPLE 1

[0021] 15.0 g of the aluminum oxide powder Martoxid MR70 (companyMartinswerk) is mixed with 10.0 g of the spinel pigment PK 3060 (companyFerro) and slurried with 52.0 g water. 70.0 g of a porous aluminum oxide(Nabalox NG100, company Nabaltec) is added thereby obtaining a highlyviscous pasty slurry. Addition of 3.8 g of a 65% nitric acid greatlyreduces the viscosity producing a stirrable suspension. 26.38 g of aninorganic binder solution (40% nano-scale zircon oxide/60% water) isadded to this suspension. The viscosity of the now sprayable suspensioncan be arbitrarily adjusted by small amounts of water and/or nitricacid.

[0022] The invention concerns a method for producing a highly porous,ceramic layer and for applying this layer onto metallic, ceramic,enameled and/or glass substrates using porous ceramic particles,preferably aluminum oxide, titanium oxide and zircon oxide and aninorganic binder system. The inorganic binder system contains at leastone ceramic nano particle of a particle size of less than 100 nm,preferably less than 50 nm and particularly preferred less than 25 nm.The solvent is water. Layers produced in this fashion are suited asself-cleaning active layers, e.g. in ovens, in combustion engines etc.or generally for coating substrates to drastically increase theirspecific surface, e.g. for catalyst substrates.

We claim: 1-17. Cancelled.
 18. A method for producing a highly porousceramic layer, said method comprising: providing a porous ceramic powderhaving an inner surface larger than 50 m²/g and an average particle sizedistribution of more than 500 nm; providing an inorganic binder systemcontaining at least one nano-scale powder and a solvent; and mixing theporous powder and inorganic binder system to form a composition.
 19. Themethod according to claim 18 wherein said solvent comprises water. 20.The method according to claim 18 wherein the porous ceramic powder innersurface is larger than 100 m²/g and the average particle sizedistribution is more than 1 μm.
 21. The method according to claim 18wherein the porous ceramic powder inner surface is larger than 150 m²/gand the average particle size distribution is more than 30 μm.
 22. Themethod according to claim 18 wherein the porous ceramic powder isselected from a group consisting of aluminum oxide, boehmite, zirconoxide, iron oxide, silicon dioxide, titanium dioxide, silicates, stonepowder, perlites, zeolites and mixtures thereof.
 23. The methodaccording to claim 18 wherein the composition comprises between 20% and80% by weight related to a solid content of the composition.
 24. Themethod according to claim 18 wherein said nano-scale powder is selectedfrom a group consisting of Al₂O₃, AlO(OH), ZrO₂, TiO₂, SiO₂, Fe₃O₂, SnO₂and mixtures thereof and wherein an average primary particle size of thenano-scale powder is below 100 nm and the solvent comprises an alcoholselected from a group consisting of 2-butoxy ethanol, ethanol,1-propanol, 2-propanol and water.
 25. The method according to claim 18wherein the composition comprises between 1% and 20% by weight ofnano-scale powder.
 26. The method according to claim 18 furthercomprising providing a third ceramic powder in an amount providingprecise adjustment of composition porosity, said third ceramic powderhaving an average particle size distribution of between about 10 nm and1 μm and said third ceramic powder is selected from a group consistingof an oxide, aluminum oxide, boehmite, zircon oxide, iron oxide, silicondioxide, titanium dioxide, silicates and stone powder.
 27. The methodaccording to claim 26 wherein the composition comprises between 5% and50% by weight referred to a solid content of the composition.
 28. Themethod according to claim 18 further comprising providing at least oneinorganic colorant and mixing the colorant with the porous powder andinorganic binder system.
 29. The method according to claim 18 furthercomprising providing a plurality of inorganic colorants in amountsenabling adjustment of color effects including patterns and spots in thecomposition, the colorant comprising spinels.
 30. The method accordingto claim 18 further comprising applying, drying and condensing thecomposition onto a ceramic, metallic, enameled or glass-like substratethrough a step selected from a group consisting of spin coating, dipcoating, immersion, flooding and spraying.
 31. The method according toclaim 30 wherein the composition is condensed at a temperature of up to1200° C.
 32. A porous ceramic layer produced in accordance with themethod of one of the claims 18 through
 31. 33. The method according toclaim 30 further comprises applying the composition to an oven surfaceto act as a self-cleaning layer.
 34. The method according to claim 30further comprising applying the composition to a medical device to actas a medicine carrier.
 35. The method according to claim 34 furthercomprising providing a bactericidal substance in the applied compositionfor bactericide applications.
 36. The method according to claim 30further comprising providing an aromatic substrate and adding saidaromatic substrate to the applied composition.
 37. The method accordingto claim 30 further comprising providing a catalyst and adding saidcatalyst to the applied composition.